A liquid crystal display device is provided with a color filter of a structure whereby, for example, picture elements of the colors red, green, and blue, which are three of the basic colors of light (hereinafter RGB color picture elements) are arranged systematically on a transparent substrate of a material such as glass, plastic, or the like.
The picture elements of a color filter are formed by means of a dyeing method in which a resin layer on which patterns have been formed using a photolithography method is dyed; a colored resist method whereby patterns are formed using a photolithography method on a previously colored plastic; a silver chloride photographic method for developing color in a silver chloride photograph; an electrodeposition method for forming patterns by electrophoresis of a coloring material; or an interference film method for forming patterns of a multilayered interference film. In addition, these picture elements can also be formed by methods wherein the above-mentioned methods are combined.
One criterion which can be given as for the image quality of a color filter for the above-mentioned type of liquid crystal display device is the contrast ratio. The contrast ratio is obtained by comparing the brilliance of a picture element dot onto which no beam is shone with the brilliance of a picture element dot onto which a beam is shone, and expressing the result as a ratio. The larger the contrast ratio of a picture, the better the picture quality. Another criterion is the variation in the spectral diffraction as discussed in Japanese Laid-Open Patent No.62-280806. In this case, the smaller the variation in the spectral diffraction, the better the image quality.
In order to increase the above-mentioned contrast ratio for the color filter, a method is known whereby a shading pattern film (black matrix) 2 is formed on the part of the picture on which no picture elements are formed, specifically, on the periphery of each of a plurality of picture elements 3 on a substrate 1, so that leakage of light is prevented, and the brilliance is reduced when no beam is shone on a picture element dot, as shown in FIG. 1.
For this type of shading pattern film, a thin film is formed on a substrate by means of a vapor deposition method, sputtering, or an ion plating method, using a metal such as chromium, aluminum, tantalum, or the like, or using a metallic oxide of copper oxide or the like. Then, the thin film is formed into the pattern film by selective removal in accordance with a photo-etching method. Or, the thin film is formed by an application in accordance with a photolithography method, a printing method, an electrodeposition method, or the like, using a coating which is a mixture of a black dye or pigment with a resin. The performance of the black matrix is evaluated by means of the reflection and shading characteristics and the like of the shading film. The shading characteristics can be illustrated by means of the optical density (extinction) value. An optical density=1 means that 10% of the incident light is passed through; an optical density=2 means that 1% of the incident light is passed through; and an optical density=3 means that 0.1% of the incident light is passed through. Specifically, the larger the numerical value of the optical density, the better the shading characteristics.
Of the conventional methods of forming the black matrix described above, in the method whereby a thin film is formed on a substrate by means of a vapor deposition method, sputtering, or an ion plating method, using a metal or a metallic oxide and then etched, a substrate for the color filter is placed in a vacuum system. For this reason, the throughput and operability are poor, and costly production equipment is required. Accordingly, it is difficult to obtain a low cost black matrix. In addition, when chromium, aluminum, tantalum, copper oxide, and the like are applied by a metal vapor deposition method, it is possible to obtain a thin film, but there is the drawback that there is considerable reflection from the transparent substrate side of the resulting thin film.
On the other hand, when a resist with light curing characteristics is used in which a black pigment such as carbon black or the like is dispersed, to fabricate the black matrix using the photolithography method (see Japanese Laid-Open Patent No. 63-298304), a thick film is required in order to increase the optical density (extinction). For example, a film thickness of 1.3 to 1.5 .mu.m is necessary to obtain the required optical density of 2.2 as a shading pattern for an STN (super twisted nematic) liquid crystal display device. Also, many such resists normally are composed of materials dissolved in organic solvents, and these are inconvenient to handle in a working environment from the aspect of safety, health, and the like.
In addition, when a black matrix is fabricated by a printing method using printing ink in which a black pigment such as carbon black or the like is dispersed in a resin (see Japanese Laid-Open Patent No. 62-153902), the sharpness of the edge of the resulting shading pattern film is poor and the surface is rough. In addition to this, a film thickness of about 1.5 .mu.m is required in order to obtain an optical density of 2.2.
From the above-mentioned conditions, conventionally, a production method by which the black matrix is attached to a substrate using vacuum vapor deposition of a metal or metal oxide, sputtering, or an ion plating method has been adopted for a black matrix for a color filter used with a thin-film transistor (TFT) liquid crystal display device for which an optical density of 3.0 or more, preferably 3.5 or more, is required. A method by which a shading pattern film is formed from resin in which carbon black or two or more types of pigment are dispersed by means of a printing methods or a photolithography method is not used in practice because a thick coated film results.
A shading pattern film for a color filter for a television picture tube has conventionally been fabricated from graphite, as disclosed in Japanese Laid-Open Patent Nos. 49-32926 and 53-18381.
However, in forming a shading pattern film for a television picture tube, a hole of about 300 .mu.m diameter is opened in the shading coated film, and preciseness is not required so highly as that in drawing a narrow line. In other words, the edge of the pattern film which has rather rough areas and areas of reduced sharpness are acceptable. Therefore, although a conventional shading pattern made of graphite can be used for a television picture tube, it is not suitable for the severe conditions required by the high detail, high sharpness in a liquid crystal display device, a solid-state image sensor, an OA sensor, and the like.